diff -urN netpbm-10.76.00-orig/converter/other/pamtopng.c netpbm-10.76.00/converter/other/pamtopng.c
--- netpbm-10.76.00-orig/converter/other/pamtopng.c	2015-07-26 09:38:56 +0900
+++ netpbm-10.76.00/converter/other/pamtopng.c	2021-01-05 13:42:53 +0900
@@ -630,6 +630,7 @@
     png_color_8 sBit;
 
     pngx_create(&pngxP, PNGX_WRITE, NULL);
+    png_set_compression_level(pngxP->png_ptr, 9);